libkafs: derivation from non-DES key (rxkad-kdf)

Add support for the "rxkad-kdf" protocol for deriving rxkad session keys
from non-DES Kerberos session keys.  This allows rxkad to be used in
realms where the KDC is unwilling or unable to issue tickets with
single-DES session keys.
Merge pull request #112 from jhutz/rxkad-kdf

libkafs: derivation from non-DES key (rxkad-kdf)